The present invention generally relates to optical recording/reproducing apparatus and particularly to an optical recording/reproducing apparatus for recording and/or playing back an information signal on and from an optical disk by means of a laser beam.
Conventionally, there is an optical recording/reproducing apparatus for recording an information signal on a rotary disk in a form of pits by irradiating same with a laser beam and for playing back the information signal from the disk by irradiating same with the laser beam so as to detect the existence or non-existence of the pits.
In such a conventional optical disk recording/reproducing apparatus, the recording is made while rotating or revolving the disk at a predetermined speed which is determined on the basis of a relation between the sensitivity of the recording medium to the laser beam and the output power of the laser diode. In the conventional apparatus, the speed of revolution of the disk is usually set to be equal at the time of recording and at the time of playback.
In such an apparatus, it is necessary to reduce the intensity of the laser beam at the time of playback relative to the case of recording so that no erroneous recording is made on the disk at the time of playback. For this purpose, the output power of the laser diode which is usually set to a first level P1 (FIG. 3) which may be 6 mW for example at the time of recording, is reduced to a second level P2 (FIG. 3) which may be 1.2 mW at the time of playback. By changing the output level of the laser diode as such, the information signal is recorded on and reproduced from the optical disk with a data transfer rate such as 2 Mbit/sec.
As far as the recording is concerned, one can achieve a satisfactory signal-to-noise (S/N) ratio or carrier-to-noise (C/N) ratio in such a conventional apparatus with respect to the noise produced in the laser diode because of the large output power. At the time of playback where the output power of the laser diode is reduced to one fifth as compared to the case of recording, however, there is a problem in that the noise caused in the laser diode which may be at least equal to the noise at the time of recording deteriorates the S/N or C/N ratio. Further, the data transfer rate is limited to a same rate such as 2 Mbit/sec for both of the recording and playback.